The present invention relates to generally to the field of porous vascular grafts. Specifically, it relates to polytetrafluoroethylene (PTFE) vascular grafts reinforced by integral helical ribs formed during extrusion of the vascular graft.
Synthetic vascular grafts are preferably characterized by being chemically inert, non-carcinogenic, non-antigenic to resist biodegradation or mechanical fatigue. Processes to make PTFE structures, including tubular structures, having a substantially uniform porous structure are exemplified by U.S. Pat. Nos. 3,953,566, 3,962,153 and 4,187,390 and Japanese Patent No. 13560/67, each of which are hereby incorporated by reference. Other processes for making PTFE vascular grafts having a porosity gradient through the tube wall are disclosed by U.S. Pat. Nos. 4,234,535 and 4,332,035. PTFE vascular grafts produced by any of the foregoing processes have received extremely favorable reception in the marketplace, but they lack resistance to kinking during, for example, joint articulation or external loading.
Radially reinforced vascular grafts are known in the art. For example, IMPRA, Inc., the assignee hereof, has for some time sold a porous expanded PTFE vascular graft having a helically wound bead of non-expanded PTFE on at least a portion of the longitudinal aspect of the graft under the trademark IMPRA FLEX and CENTERFLEX. U.S. Pat. No. 4,588,461 discloses a process for producing a vascular graft in which a tubular graft is mounted on a mandrel and axially rotated while a reinforcing thread is wound substantially tensionless on the tubular prosthesis. The reinforcing thread is adhered to the tubular graft by a solvent specific for the material of the reinforcing thread. A spacer band, insoluble in the thread solvent, is used as a carrier for the reinforcing thread. The wound graft is heated to remove the solvent and the spacer band is removed leaving the helically wound vascular graft. A similar type of wound bead is disclosed in U.S. Pat. No. 5,061,275. U.S. Pat. No. 4,969,896 discloses a reinforced vascular graft having discrete longitudinally oriented, radially spaced ribs on the outer peripheral surface of the tubular graft. The ribs are preferably made of solid silicone rubber. A sleeve or wrap is placed over the prosthesis such that it bridges between adjacent ribs defining spaces between the tubular graft and the wrap. The wrap is adhered to the graft by applying an adhesive bead. The tubular graft and the ribs are made separately as discrete components then the ribs are adhered to the outer peripheral surface of the tubular graft. These types of reinforced vascular grafts suffer from one or more difficulties, including, peeling or delamination of the reinforcing structure from the tube, compression of the unsintered graft which weakens the graft at the point of contact between the reinforcing structure and the tube, inability to suture the reinforcing structure, requiring removal of the reinforcing structure at the suture point, and potential contamination introduced by application of the reinforcing structure to the tube wall.
Moreover, none of the foregoing reinforced vascular grafts have microporous reinforcing structures. U.S. Pat. Nos. 4,550,447 and 4,647,416 disclose a ribbed porous vascular graft and a method for preparing a ribbed porous vascular graft, respectively. These patents teach that an expanded or unexpended PTFE tube may be scored in a regular and repeating pattern. The scored tube is then expanded below the sintering temperature of PTFE, thereby separating the scores into ribs, then sintering the scored, expanded tube while restraining the tube from axial shrinkage. The resulting graft creates ribs having a porosity less than that of the tube wall because the ribs are isolated from axial tension during sintering.
Heretofore, however, the prior art is devoid of a porous expanded reinforced PTFE tubular vascular graft in which the reinforcing structure exhibits substantially identical porosity as the tubular graft itself.